Precise liquid handling is usually performed using pipettes. Pipettes are commonly used in molecular biology, analytical chemistry and medical tests. Pipettes come in several designs for various purposes with differing levels of accuracy, from single piece glass pipettes to more complex adjustable or electronic pipettes. Many pipette types work by creating a partial vacuum above the liquid-holding chamber and selectively releasing this vacuum to draw up and dispense liquid.
Pipettes that dispense between 1 and 1000 micro liter are termed micropipettes, while macropipettes dispense a greater volume. Two types of micropipettes are generally used: air-displacement pipettes and positive-displacement pipettes. In particular, piston-driven air-displacement pipettes are micropipettes which dispense an adjustable volume of liquid from a disposable tip.
FIG. 1 shows the outside of a known pipette 1 with a pipette body 3, a tip 5, a piston 7. The pipette body 3 contains a plunger (not shown) inside, which provides suction to pull liquid into the tip 5 when the piston 7 is compressed and released. The maximum displacement of the plunger is set by a dial 9 on the pipette body 3, allowing the delivery volume to be changed.
Larger capacity tubular pipettes, such as volumetric or graduated pipettes, are used by temporarily attaching a pipetting dispenser. Pipetting syringes typically handle volumes in the 0.5 mL to 25 mL range. Micropipettes use disposable tips to avoid contamination of samples.
Pipettes working with disposable tips 5 are usually micro pipettes. Tips are mostly made from polypropylene, because of its inertness in chemical reactions, it's resistance to chemical compounds and it's flexibility. This flexibility is necessary to provide an airtight seal between the pipette tip 5 and the pipette body 3. When using a harder material for the pipette tip 5, a softer, flexible insert (not shown) can be used in the tip 5 on the pipette side of the pipette tip 5, to provide the airtight seal between the pipette tip 5 and the pipette body 3. It is also possible to use a pipette with one or more sealing o-rings of suitably soft material to seal any space between the pipette body 3 and the tip 5.
In drawing up liquid from an open vessel, such as an opened Eppendorf tube or micro titer plate, flexibility is no problem. When loading a very narrow gel (for example a sequencing gel) flexibility may become a problem, as tips need to be ultra thin at the drawing/dispensing end to be able to fit between the glass plates holding a very thin gel. Tips made from a flexible material, such as polypropylene will not be rigid enough to keep their shape. In this case polycarbonate may be used (Drummond-Sigma).
In molecular biology, nucleic acid amplification techniques such as PCR (Polymerase Chain Reaction) are used for amplification of short polynucleotide sequences of RNA or DNA (up to 1000 nucleotides, but occasionally longer, up 10.000 nucleotides or even longer). The PCR process has been performed for the first time in 1989 by Kary Mullis. Another example of such a process is NASBA (Nucleic Acid Sequence-Based Amplification).
When performing PCR in a sealed enclosure, made from a suitable plastic foil (PCT/NL2011/050354, unpublished), it may be cumbersome to retrieve the sample from the enclosure. By providing a holding block, having the negative form of the foil with enclosures, the enclosures may be fixed in a block for further processing. Micro titer plates sealed by adhesive films (plate sealers) can be used without negative blocks as a holder. Several means can be used for opening the enclosures. With a knife (for example a scalpel) or another sharp object from a suitable material the enclosures may be opened, making the liquid sample in the enclosures available for aspiration by a pipette. Cross contamination is a danger. Alternatively the samples can be drawn up by a syringe, holding a sharp needle. Usually both syringe and needle must be disposed of after drawing up one sample, to avoid contamination of following samples, making this a costly procedure. Sealed micro titer plates can be opened by removing the adhesive plastic foil (plate sealer).